1. Field of the Invention
The field of the invention pertains to drilling fluid additives that may be used to remediate bit balling that otherwise occurs in clay and shale (hereafter referred to collectively as ‘clay’) zones as a well is drilled.
2. Description of the Related Art
Water-based drilling mud systems are usually employed during the drilling of a well, such as an oil or gas well. The well bore may penetrate a clay zone, and this circumstance exposes naturally occurring clay minerals. Water in the mud is able to hydrate the clay minerals, which then typically swell to cause a number of known problems.
The clay problems may be addressed by using chemical treating agents. This is done using salts and/or polymers in the drilling mud. Salts, such as KCl or CaCl, reduce the clay swelling and dispersion. Such polymers as partially hydrolyzed polyacrylamide are sometimes added to the drilling mud used to encapsulate the clay, thus keeping the clay from hydrating and swelling. Alternatively, oil-based mud may be used to prevent the swelling phenomenon, but since oil mud also contains water, it is necessary that the salinity of the water be sufficiently high to prevent water from entering the clay by osmosis.
Bit balling is one problem that is frequently encountered when drilling through clay. The problem is caused by the tendency of hydrated clay minerals to stick or adhere to the bit and bottom-hole assembly of a drill string. Although this problem may also occur in oil based mud, it is relatively rare as compared to water-based mud. From an operations standpoint, bit balling is evidenced by increased pump pressures as the flow pathway through the well bore annulus becomes blocked, reduced rates of penetration, blocked shaker screens, a required over-pull tension that occurs due to a restricted annulus when tripping pipe, and possible stuck pipe.
Once bit balling is diagnosed, conventional methods of remediation are to increase the weight on the bit, add chemicals and perhaps pull the drill pipe out of the hole to clean the bit and bottom hole assembly. For a water-based mud, a detergent may be added to the drilling mud to reduce the ability of the hydrated clay to stick to the bit and bottom hole assembly. Glycol may also be added at about 3% to 4% of system volume. This often fails to cure the problem.
Preventative measures against bit balling include the review of prior drilling reports to ascertain and adopt procedures that have previously worked in the geographic area to overcome bit balling. A KCl/polymer or CaCl/polymer mud may be used to inhibit the swelling of clays. The selection of a bit may also affect bit balling, where it is known that polycrystalline diamond compact bits are more prone to balling than are tri-cone bits, and it is further the case that the arrangement of teeth structures on tri-cone bits may affect bit balling. This is shown for example, in U.S. Pat. No. 4,984,643 issued to Isbell et al.
Another way to address the problem of bit balling is to optimize the mud system hydraulics. This may be done for either large or small bore bits. In addition to sizing the nozzles for optimum delivery of hydraulic horsepower, it is also possible to direct the nozzle discharge to optimize bottom-hole cleaning in a ‘mud pick’ configuration. This is reported, for example, in Smith et al., Hydraulics Optimization Research in Large Diameter Bits Reduces Operator's Variable Costs, AASDE-05-NTCE-58 (2005).
It has also been reported that maintaining a negative potential of a few volts on the drill string assembly may liberate water at the interface between the bit and the hydrated clay. See Sanjit et al., The effect of electro-osmosis on the indentation of clays, Proceedings of the 32nd US Rock Mechanics Symposium, Norman Okla. (July 1991).
Although the art does provide remedial measures, bit balling continues to be a significant factor affecting the costs of drilling new wells. It is particularly difficult to pump pills of material to remediate the problem of bit balling, and any such measures often provide only temporary relief.